1. Technical Field
The present application relates to the field of optical communications, and in particular, to an optical fiber grating tracker and a method for detecting an optical fiber line fault.
2. Related Arts
An optical fiber grating is an optical fiber passive device, and actually an optical fiber, where a fiber core of the optical fiber has a structure having a refractive index changed periodically or called a Bragg reflector in the optical fiber core. By means of ultraviolet light sensitivity of an optical fiber material, through methods such as a two beam interference method and a phase mask method, a bare fiber is exposed from the side surface to an interference pattern of an ultraviolet light beam, so as to write the interference pattern to the optical fiber and form a space phase grating inside the fiber core. After an optical signal with a specific spectrum width passes through the optical fiber grating, an optical wave with a specific wavelength is reflected along the original path, and optical signals with other wavelengths are transmitted directly. According to the mode coupling theory, a wave with a wavelength of λB=2 nΛ is reflected by the optical fiber grating (λB is a center wavelength of the optical fiber grating, Λ is a grating period, and n is an effective refractive index of the fiber core). The reflected center wavelength signal λB is related to the grating period Λ and the effective refractive index n of the fiber core. The reflected wavelength λ is changed as factors such as the external temperature and the stress change.
The optical fiber grating belongs to a reflection-type work device. When continuous broadband light emitted from an optical source is incident through a transmission optical fiber, a coupling effect occurs between the light and an optical field, corresponding narrowband light is selectively reflected for the broadband light and returns along the original transmission optical fiber, and other broadband light is directly transmitted.
The optical fiber grating has the advantages such as anti-electromagnetism, anti-corrosion, high temperature resistance, not having electricity, not generating heat, flame-proof, anti-explosion, having light weight, having small volume, being capable of running safely in a harmful or dangerous environment. With the rapid increase of social information demands, as one of main pillars in the information field, the optical communication faces new challenges continuously, so the optical communication needs to be continuously updated and improved, so as to adapt to the rapid development of the information society. The optical fiber grating is already applied to aspects such as a laser source, an optical amplifier, optical signal processing, wavelength division multiplexing, optical adding/dropping, and optical filtering in the optical communication field. The optical fiber grating may be used to manufacture a large-power optical fiber laser, a narrowband laser, and a tunable laser. The optical fiber grating may be used to manufacture a gain flattening filter for gain balance of an EDFA. The optical fiber grating may be used to manufacture an optical fiber dispersion compensator. The optical fiber grating may be used to manufacture a dense wavelength division multiplexing device and a network adder/dropper. The optical fiber grating may be used to implement ultra-narrowband filtering.
In an optical fiber network, damage and fault location of the optical fiber mainly relies on an Optical Time Domain Reflection (OTDR) technology. The principle of the OTDR technology is similar to that of the existing ultrasonic reversing radar.
It can be known according to the electromagnetic field theory that, due to affection of factors such as microscopic density changes and constituent fluctuations of the fiber core medium material, an incident photon and a medium molecule interact with each other, so as to generate Rayleigh scattering with the same frequency as the incident light, and moreover, a nonlinear collision occurs between the incident photon and the medium molecule due to a nonlinear effect of the medium. In an inelasticity process, the photon and the molecule exchange energy, a motion direction of the photon changes and meanwhile a part of energy of the photon is delivered to the molecule, or energy of vibration and rotation of the molecule is delivered to the photon, so as to change the frequency of the photon. This process is called Raman scattering.
When an optical pulse is transmitted along the optical fiber, each point of the optical fiber generates Rayleigh scattering, where the scattering is isotropic, and a part of scattering light returns along the optical fiber. If time begins to be counted at the moment when the optical pulse enters the optical fiber, a scattered echo signal received at an injection end at a different point of time t is characterized in that, the signal is generated by an optical fiber at a distance of L from the injection end:
  L  =      ct          2      ⁢      n      
In the equation, t is a point of time when the optical pulse returns, L is a scattering position of the optical fiber, c is a light speed in vacuum, and n is a refractive index of the optical fiber core.
It can be seen from the above equation that, once the optical fiber is determined, the refractive index of the optical fiber is also determined, and a light transmission speed in the optical fiber is determined accordingly. The transmission speed is 200,000 kilometers per second, and the roundtrip time is added, so as to implement space location of the optical fiber network through the OTDR technology. This method for damage and fault location of the optical fiber is the unique detection method in the current optical network, but the method is only applicable to a single optical fiber. If an optical fiber branch exists in the network, the OTDR cannot perform damage and fault location of the optical fiber branch.